A known surface acoustic wave (SAW) device comprises a substrate of piezo-electric material having two input transducers and two output transducers on its upper surface. Each transducer has a generally rectangular contact pad, to which electrical contact is made by wire bonding. The four pads are arranged in a rectangular array, rather than being collinear.
SAW devices are made using several of the techniques that are employed in the manufacture of integrated circuits, and for this reason it is conventional to fabricate a large number of SAW devices, e.g. 100 or more, concurrently on a wafer or substrate material and then dice the wafer and package and individual SAW devices. The SAW devices could be tested after dicing and packaging, using the bond wires to achieve electrical connection to the devices. However, for reasons that are well understood by those skilled in the art, it is desirable that the SAW devices be tested in wafer form, i.e. prior to dicing.
A wafer prober developed for use in testing integrated circuits in wafer form comprises a stage or table defining an X-Y plane on which the wafer is placed, a probe head having probe tips for engaging the contact pads of an individual integrated circuit, and a drive mechanism for moving the probe head in the X, Y and Z directions. The drive mechanism is programmed with respect to the pitch of the integrated circuits in the X and Y directions, and on initial set-up the probe head is positioned in the Z direction so that the probe tips engage the contact pads of a given integrated circuit with a force that is considered by the operator of the machine to be acceptable. This position establishes the Z axis adjustment of the prober. In use, the prober drive mechanism automatically advances the probe head from die to die by raising the probe head, moving it in the X and/or Y direction, and lowering it in the Z direction to the position established on initial set-up.
The conventional wafer prober has no means for determining the force with which the probe tips are applied to the integrated circuit contact pads. If the operator makes an error on initial set-up, the contact pressure might be too high, resulting in the wafr and/or the probe head being damaged. If the conventional wafer prober is used in the testing of SAW devices, the absence of reliable means for measuring contact force implies that the contact force will vary from wafer to wafer, and may vary from die to die if the upper surface of the wafer does not lie accurately perpendicular to the Z axis. Since SAW devices are pressure-sensitive, variation in contact force may result in spurious measurement results.
Conventional integrated circuit dice are rectangular in form and may have 20 or more contact pads along each of the four sides. The contact pads along a given side are in a linear array. Conventional wafer probers for integrated circuits employ four probe heads, associated with the four sides respectively of a die. The probe head described in U.S. patent application Ser. No. 672,846 filed Nov. 16, 1984 comprises a platelike support member of ceramic material, the support member being sufficiently thin to bend somewhat. The probe tips of each probe head are arranged in a linear array and may be, for example, gold beads. The malleability of the gold and the limited flexibilty of the support member allow contact to be made to each of the contact pads with a reasonable degree of reliability. In a SAW device, in which there are relatively few contact pads, it is desirable to use a single probe head, but because the contact pads are not collinear the flexibility of the support member cannot compensate for possible deviations of the plane containing the probe surfaces of the probe tips from its nominal orientation. Moreover, whereas the contact pad material of conventional integrated circuits is frequently gold, it is common to use aluminum for the contact pads in a SAW device, and therefore it is desirable that the probe tips be made of a hard, non-malleable metal, such as nickel, in order to penetrate the passivation layer that forms on the aluminum contact pads.